Worldwide variations in melanoma rates among whites have shown that rates increase with increasing proximity to the equator. The relationship between latitude and malignant melanoma rates was explored in California, a large state covering almost 10 degrees of latitude. A total of 14,653 newly diagnosed cases of invasive melanoma was reported for white, non-Hispanic residents during the first five years of population-based statewide cancer reporting in California (1988-92). Latitude, climate zone, and urban/rural status were assigned to cases based on census tract of residence at diagnosis. Rate ratios were estimated using multivariate Poisson regression modeling, controlling for age and sex. For each degree of latitude the rate ratio was 0.98 (95% confidence interval: 0.97, 0.99), indicating decreasing rates with increasing latitude (greater distance from the equator). The melanoma rate in the most southern latitude of the state was 70% higher than in the northern most area. However the relationship between latitude and melanoma rates was not linear and was explained largely by urban and rural differences. The state's major urban areas, located in the south and middle of the state, had the highest rates. Striking climate zone variations that exist within the state appeared to explain rate differences better than did latitude and independent of urbanization. Once taking into account variations in urbanization and microclimates, latitude did not appear to be strongly associated with melanoma rate differences.